This function retrieves the Issuer Alternative Name (2.5.29.18),
contained in the given certificate in the X509v3 Certificate
Extensions.

When the SAN type is otherName, it will extract the data in the
otherNames value field, and GNUTLS_SAN_OTHERNAME is returned.
You may use gnutls_x509_crt_get_subject_alt_othername_oid() to get
the corresponding OID and the "virtual" SAN types (e.g.,
GNUTLS_SAN_OTHERNAME_XMPP).

If an otherName OID is known, the data will be decoded. Otherwise
the returned data will be DER encoded, and you will have to decode
it yourself. Currently, only the RFC 3920 id-on-xmppAddr Issuer
AltName is recognized.

the alternative issuer name type on success, one of the
enumerated gnutls_x509_subject_alt_name_t. It will return
GNUTLS_E_SHORT_MEMORY_BUFFER if ian_size is not large enough
to hold the value. In that case ian_size will be updated with
the required size. If the certificate does not have an
Alternative name with the specified sequence number then
GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE is returned.